1. Field of the Invention
The present invention relates to a hub unit for sensing rotational speed, and, in particular, to a hub unit which is installed in a built-in Antilock Brake System (ABS) or Traction Control System (TCS) and is used to support the wheels of the automobile in a freely rotatable manner with respect to a suspension system and to sense the speed of rotation of the wheels.
2. Description of the Prior Art
The wheels of an automobile are supported in a freely rotatable manner through a suspension device, and it is essential that the speed of rotation of the wheels be sensed in order to control an Antilock Brake System (ABS) or Traction Control System (TCS). Conventionally, a hub unit for sensing rotational speed, as shown in FIG. 13, has been used for this purpose.
This hub unit for sensing rotational speed comprises a hub 3, an inner ring or race member 4, a nut 6, an outer ring or race member 9, and a plurality of rolling members 10.
The hub 3 is provided with an axially outer end (the left end in FIG. 13) having a flange section 1 for securing the wheel, a middle section and an axially inner end section (the right end section in FIG. 13).
A radially outwardly facing inner ring track 2a is formed on the radially outer peripheral surface of the middle section of the hub 3. The inner ring member 4 is externally secured or fitted on the radially outer peripheral surface of the middle section of the hub 3 and provided with a radially outwardly facing inner track 2b on its radially outer peripheral surface.
The nut 6 is placed in threaded engagement with the male threaded section formed on the radially outer peripheral surface of the inner end section of the hub 3. The nut 6 is tightened to press against the axially inner end surface of the inner ring member 4, and the inner ring member 4 is secured to the radially outer peripheral surface of the hub 3 in a specified position.
The outer ring member 9 is provided with a mounting section 7 on its radially outer peripheral surface to provide support on a suspension device (omitted from the drawing) and a plurality of inwardly facing outer ring tracks 8a, 8b are formed on the radially inner peripheral surface of the outer ring member 9.
The rolling members 10 are provided between the hub 3 and the outer ring member 9, and between the inner ring member 4 and the outer ring member 9.
The hub unit for sensing rotational speed is thus comprised of the hub 3, the inner ring or race member 4, the nut 6, the outer ring or race member 9, and the plurality of rolling members 10 to support the hub 3 in a freely rotatable manner on the inside of the outer ring member 9 which is supported on the suspension system.
The nut 6 is formed with a irregular section 11 on the axially inner end surface thereof. The irregular section 11 of the nut 6 functions as a pulse rotor which generates a pulse as the hub 3 rotates.
A cover 12 is mounted at an axially inner end section (the right end section in FIG. 13) with an opening of the outer ring member 9 and covers the opening of the outer ring member 9. A sensor 13 is secured to the cover 12. The axially outer end surface (the left end surface in FIG. 13) of the sensor 13 faces the irregular section 11 of the nut 6.
Through the rotational speed sensing hub unit described above, a wheel secured to the flange section 1 provided on the axially outer face section of the hub 3 is supported in a freely rotatable manner by the suspension system which supports the outer ring member 9.
The output of the sensor 13 varies according to the rotation of the wheel. The frequency of the variation of the output of the sensor 13 is proportional to the rotational speed of the wheel. For this reason, the rotational speed of the wheel is obtained by inputting the output signal from the sensor 13 to a controller (not shown). The ABS and the TCS are then suitably controlled according to the desired speed of rotation.
A distance 1 between the part which functions as the pulse rotor on the rotational speed sensing hub unit (the irregular section 11 formed on the axially inner end surface of the nut 6 in the example shown in FIG. 13) and the axially outer end surface of the sensor 13 must be precisely controlled to a suitable value for accurate sensing of the rotational speed.
The nut 6 which functions as the pulse rotor in FIG. 13 plays the role of securing the inner ring member 4 to the radially outer peripheral surface of the hub 3. This nut 6 must be firmly tightened on a male threaded section 5 of the radially outer peripheral surface of the inner end section of the hub 3. For this reason it is difficult to precisely control the positional relationship between the nut 6 and the hub 3 and the outer ring member 9.
Specifically, a suitable value for the distance 1 is an extremely small value of less than 1 mm. For this reason, the distance 1 cannot be maintained if the positional relationship between the nut 6 and the hub 3 and the outer ring member 9 deviates by even a very small amount. If the distance 1 exceeds a suitable value, there is concern that the ABS and TCS cannot perform adequately.
In particular, in the structure shown in FIG. 13, there is no method for determining whether or not the distance 1 after the hub unit is assembled is the correct value other than by checking the output of the sensor 13. However, the distance 1 cannot be adjusted during assembly. For this reason, highly precisely fabricated parts are required for the hub to maintain a suitable value for the distance 1, resulting in the high cost of manufacturing the hub unit.